(1) According to the conventional methods of mass spectrometric analysis, the measurement target is ionized, the ions formed are sent to a mass spectrometer, and the mass numbers of the dissociation products are measured. In a tandem type mass spectrograph in which multistage dissociation is possible, an ionic species having a certain mass number alone is selected from among the ionic species formed by the dissociation reaction and is further caused to collide with gas molecules.
In this manner, second-stage, third-stage, . . . , and nth-stage dissociation reactions are induced, and the mass numbers of the ionic species formed in each stage are measured. In this case, the ionic species to be dissociated in each of the second and subsequent stages is generally selected according to the findings obtained by the measurer.
(2) JP-A-2000-171442, for instance, may be mentioned as a prior art document dealing with the selection of an ionic species to be measured. In the patent document, mention is made of a method of selecting that ionic species which shows the highest spectral intensity. Further, the method comprising selecting some species high in spectral intensity or selecting the one k-th (k being selected by the measurer) in spectral intensity is used in some instances as a method generally employed.
(3) Generally, use is made of the method comprising matching the spectrum measured with a database in which spectral data on structurally known polypeptides as collected in advance are stored, and thus structurally identifying the polypeptide in question. For compounds other than proteins, JP-A-H05-164751 (1993) is concerned with the structural identification thereof utilizing a database.
(1) In carrying out the n-th stage dissociation (hereinafter referred to as “MSn”) according to the prior art methods, the ionic species to be subjected to MSn is selected based on the measurer's findings from the dissociation spectrum obtained in the (n−1)th stage (MSn−1). Therefore, the MSn measurement is troublesome and, generally, the spectral analysis is made only to the stage of n=2 in many instances. At the stage of n=2, no sufficient spectral information necessary for the purpose of identification may be obtained in some instances.
(2) The above-cited Patent Document 1 is concerned with the establishment of optimum analysis conditions, hence cannot always be said to be best advisable from the viewpoint of improving the precision in identifying biopolymers, in particular polypeptides.
Further, when the ion selection is made based on the intensity information, there arises the possibility of failure in selecting the optimum ion for obtaining the structural information. It is necessary to effectively utilize the mass-to-charge ratio (m/z) values of the ions formed.
(3) Supposing that the number of amino acid residues constituting a peptide chain is K and the number of amino acid species is 20, the number of possible amino acid sequences becomes as large as K20. If chemical modifications of amino acid side chains are taken into consideration, that number will become still larger.
Therefore, it is almost impossible to prepare a database taking chemical modifications into consideration and carry out searching within a practical period of time.
On the other hand, for chemical modification group elimination, a chemical pretreatment is necessary, and this may cause a decrease in measurement throughput. The database-based matching software currently available on the market has a problem in that only measurements until MS2 can be dealt with.
For solving the problems discussed above, it is necessary to select an optimum ionic species in each stage of MSn (n≧3) and thereby effectively utilize the information contained in the MSn spectrum.